Lecture 27 : Experimental methods in chemical kinetics
For the two dissociation products (covalent : Na + I and ionic: Na+ + I - ) there are two surfaces V0 and V1. The two surfaces "cross" near 6.9
. In this case, crossing from one surface to another is avoided. This region of avoided crossing is also the transition state. Beyond this distance, the molecule can be considered to be dissociated.
When the molecule is vertically excited from its minimum energy in V0, it gives to the surface V1 and can oscillate in the upper V1 surface for a few vibrational periods. The progress of the reaction can be studied by a probe pulse of wavelength near 589 nm (corresponding to the sodium D absorption). It is observed that the fluorescence intensity does not rise smoothly but there are several undulations of a period about 1.2 ps (Fig 27.7)
Figure 27.7 Fluorescence intensity of Na and NaI as a function of time.
At 589nm, the fluorescence intensity of Na grows to the largest value after about 8 ps. The lower curve shows the fluorescence of bound sodium in NaI. The molecule undergoes 6 to 7 vibrations before the covalent dissociation is complete. The rise and fall in intensity is due to the NaI molecule (which is represented as a wave packet in Fig 27.7) going back and forth in the upper curve V1.
. In this case, crossing from one surface to another is avoided. This region of avoided crossing is also the transition state. Beyond this distance, the molecule can be considered to be dissociated. 
